Q-switching apparatus for a laser device



m 3,311,.a45 I I Mam}?! E57 c. J. KOESTER 3,311,845

Q-SWITCHING APPARATUS FOR A LASER DEVICE Filed June 10, 1963 2Sheets-Sheet l CONTROL 1 F /6 N /h Z 27 29 /8- -/9 22 3/ 32 POWER /7PULSE SOURCE POTENTIAL SOURCE 53 INVENTOR.

Char/es J Kaesfer QQA Q 24 A ffomey March 1957 c. J. KOESTER 3,31L84z5Q-5WITCHING APPARATUS FOR A LASER DEVICE Filed June'lo, 1963 2Sheets-Sheet 2 I NVENTOR Char/65 J.

Affomey Q-SWITCHING AFPARATUS FOR A LASER DEV C This invention relatesto laser structures, and in particular to resonant laser structures ofthe type wherein energy-loss factors can be. varied to control ormodulate the laser light output.

Lasers are devices for producing coherent, monochromatic light of veryhigh intensity and narrow beam spread. A typical laser structureincludes a body of laser material arranged Within a resonant cavitydefined between opposed internally reflective surfaces at least one ofwhich may be partially light-transmissive. Light is produced in thelaser by photonic emission occurring incident to the transition ofactive atoms in the laser body from an excited, upper energy level to aterminal low energy level.

By way of specific illustration, to initiate laser operation in aconventional solid-state laser device the laser body is pumped with aninput pulse of light energy of appropriate absorptive wavelength. Thepumping light excites active atoms in the body to shift from an initiallow energy level to the aforementioned upper level through a series ofinterlevel. transitions, with the result that the population ofupper-level atoms in the body becomes greater than the remaininglower-level population, a condition termed an inversion of energystates. Light emitted by spontaneous emissive transitions of individualup, er-level atoms in the body then reflects back and forth through thebody between the cavity ends, and induces similar light-emissivetransitions of other upper-level atoms in such manner as to create afast-rising bidirectionally reflected pulse of coherent light in thecavity. A portion of this pulse emerges through the partiallytransmissive cavity end to constitute the laser light output.

Such light emission occurs when the difference between the upper-levelpopulation and the lower-level population in the laser body equals orexceeds a so-called threshold value, which is directly related to thetotal magnitude of energy-loss factors in the resonant cavity structure.Accordingly, the pumping energy input to the body must be sufiicient toestablish a degree of inversion therein equal to or greater than thethreshold value. Once the latter condition is established, laser actionproducing one or a succession of output light'pulses ordinarilycontinues until the degree of inversion in the body falls again belowthe threshold value due to depletion of the upper-level opulation bylight-e1nissive transitions of u er-level atoms.

Since the threshold value of inversion requisite for laser actiondepends on the energy-loss factors in the cavity, the initiation andduration of the laser light output can be controlled by varying thetotal magnitude of these factors. Specifically, an increase inenergy-loss factors at the outset of the pumping period can raise thethreshold requirement of the cavity so as to prevent or delay theinitiation of laser action notwithstanding the establishment of arelatively large degree of inversion by the pumping light input. Again,an increase in energyloss factors at a given moment during laser actioncan raise the threshold requirement above the existing degree ofinvcrsion in the laser body so as to interrupt or terminate the laserlight output at the given moment. Such controlled variation ofenergy-loss factors is referred to as Q control since it changes the Qor quality factor of the cavity, which is a value directly proportionalto the ratio of wave energy storage to wave energy dissipation per waveenergy cycle in the cavity.

trite-d rates Patent: mce

Patented Mar. 28, 1967 The modes of propagation in which stimulatedemission can occur are also determined by energy-loss factors in thecavity. Individual excited atoms in the laser body may spontaneouslyemit light in any of a very large number of modes, including modes forplane Waves propagating parallel to the long axis of the body and modesfor waves propagating at various angles to the latter axis. To theextent that light emitted in any particular mode induces emissivetransitions of other upper-level atoms, the induced emission occurs inthe same mode. However, significant stimulated emission (providing alaser light output) does not develop in modes ror which energy-lossfactors in the cavity are high, since light in the la ter modes is notreflected repeatedly back and forth through the body but is quicklydissipated (due to such energy-loss factors) before it can inducemassive emissive transitions of other atoms. Thus, for example, ifenergy-loss factors in the cavity are controlled to provide high energylosses for modes for waves propagating at any angle to the laser axisgreater than a given angle, the laser light output will be restricted tomodes for waves propagating at angles smaller than such given angle. Oneresult of this mode selection is to control the beam spread of theoutput light; in general, the magnitude of the output beam spread angleis directly related to the magnitude of the aforementioned given angle.

The present invention comprises a resonant laser structure incorporatinga laser body in a resonant structure cflective to control the laserlight by variations of energyloss factors in the laser resonant cavity.In various embodiments of the invention as described below, the outputmay be controlled in several ways, for example with respect toinitiation and duration (by switching the output on or off), or withrespect to beam spread angle, by modeselectivc changes in energy-lossfactors. While the structure of the invention will be understood to havea variety of applications, it is particularly suitable for lingmodulation of the laser emission by means of Q control so that the laseroutput light beam may be used to trans mit information, for example incommunications and data processing systems. Use of laser emissive energyas a carrier wave affords particular advantages, among which are itsrelatively Wide frequency-band characteristics and the extraordinarilyhigh intensity and narrow beam spread of the laser output.

The resonant structure of the present invention, including the laserbody, defines a light-propagation path extending between resonant cavitytermini. A propagation path portion of the path-defining structure,formed of a first material transmissive to laser emissive light energy(and having for such light energy a refractive index herein designatedin), is engaged externally over a smooth surface area by a secondmaterial (herein termed a cladding) also transmissive to such lightenergy (and having a refractive index herein designated 11 One (or both)of the aforementioned materials exhibits a Kerr or Faraday effect and achange in its refractive index under the influence of an appliedelectric or magnetic field; specifically, such change in refractiveindex occurs within a range of indiccs for which the ratio 712/11 variesbetween a value less than unity and a higher value preferably equal toor greater than unity. The resonant structure is so arranged that lightemitted in the laser body and propagating along the path through thefirst material (which may be an integral portion of the laser body)impin es obliquely on the interface between the latter and the secondmaterial.

If the ratio 712/111 is less than unity, so that the index of the secondmaterial is lower than the index of the propagation path portion formedby the first material, the interface is totally reflective to lightpropagating through the first material and impinging obliquely on theinterface at an angle of incidence greater than a given critical angle0,, which is determined by the equation This condition, commonlyreferred to as total internal rettcction, inhibits dissipation of lightenergy from the resonant structure through the interface, and therebyenables attainment of a low-energy-loss or so-called high- Q conditionin the structure, permitting development of laser action. If, however,the ratio n /n is increased to a value which equals or exceeds unity sothat the index n of the second material equals or exceeds that of thefirst material of index 11 the interface no longer ha total internalrellection but instead transmits at least a substantial portion of thelaser emissive energy impinging thereon, being most fully transmissivewhen 11 equals n The resultant increase in energy losses thus effectedin the structure is sutficiently large to prevent or terminate laseraction.

Consequently Q control is accomplished in the present invention by meansof appropriate field-creating structure arranged for application of theindex-changing field to the material (or materials) of variable index.For example, if application of the field increases the value of theratio 12 /11 control of the initiation and duration of the laser lightoutput involves energizing the field-creating structure to apply a fieldeffective to increase such ratio to a value at least equal to unity whenit is desired to interrupt laser action, and deenergizing the latterstructure to remove the field when it is desired to permit laser action.Since the response time of the Kerr or Faraday cladding material to theapplication of the field is very short, the control effect isconcomitantly rapid, facilitating pulse or other high frequencymodulation of the laser output by control of energization of the fieldcreating structure by a signal representative of information to betransmitted.

With an appropriate arrangement of elements, modeselective variation inenergy-loss factors providing control of the laser output beam spreadangle may similarly be effected by varying the ratio 11 /11 in theforegoing manner within the range of values below unity. For instance,the structure of the invention may be designed so that the angle formedbetween any given direction of light propagation and the laser axis iscomplementary to the incident angle of such direction at theaforementioned interface. In this case, there is total internalreflection at the interface only for light diverging from the laser axisat an angle smaller than the complement t of the critical incident anglebecause such total internal reflection exists only for light strikingthe interface at an incident angle greater than o Hence there is ahigl1-loss condition at the interface, preventing significantdevelopmentcf stimulated emission in the laser body, for modes for wavespropagating at an angle to rthe laser axis greater than o The beamspread angle of the laser output is then directly dependent on themagnitude of the angle and since the latter angle is decreased withincrease of the magnitude of the angle 0 an increase in the value of theratio {lg/H1, by increasing the magnitude of 6 concomitantly reduces theoutput beam spread angle.

Further features and advantages of the invention will be apparent fromthe detailed description hereinbelow set forth, together with theaccompanying drawings, wherein:

FIG. 1 is a schematic sectional view of a laser structure embodying thepresent invention in a particular form;

FIG. 2 is a schematic view, partly in section, of an alternativearrangement for the structure of FIG. 1 including a modified form offiber laser structure;

FIG. 3 is a sectional view taken along the plane 3-3 of HG. 2;

FIG. 4 is a schematic sectional view of another embodiment of theinvention;

FIG. 5 is a schematic sectional view of a further alternative embodimentof the invention;

FIG. 6 is a schematic sectional view of a modified form of the structureof FIG. 5; and

*IG. 7 is a Schematic sectional view of another embodiment of theinvention.

Referring first to FIG. l, the resonant structure of the device thereshown is provided by an elongated laser element or fiber 10, comprisinga body of solid laser material of elongated, smooth-sided configuration(as in the form of an optical fiber) having internally rctlecthecavity-terminating end faces. An 0 tcal fiber is a fibershaped body oflight-transmissive material providing an elongated light propagationpath of relatively small crosssectional area. The boundaries of the pathare established at the smooth, elongated peripheral surface of thefiber, which is ordinarily surrounded by material (such as air or asuitable cladding) having a refractive index lower than that of thefiber material to provide total internal reflection at the interface;light propagating through the fiber impinges obliquely on this boundarysurface and is successively reflected from side to side of the fiber bythe latter surfac Hence, in a fiber laser, attainment of a high Qcondition is dependent on such total internal reflection, whichminimizes dissipation of laser emissive energy through the fiber sidewalls and concentrates the energy in the fiber. Q control for such astructure may therefore be provided, in accordance with the present invention, by cladding a preselected length of the fiber boundary surfacewith a suitable Kerr or Faraday material, to vary the internalreflectivity of such surface portion in the manner described above.

The element or fiber 10 of FIG. 1 is an unclad fiber laser core oftypically thin, elongate cylindrical configuration, fabricated of asuitable solid laser material such as neodyminurn-doped glass, with sidewall surfaces exposed to the air. The plane, axially perpendicular,opposed end surfaces 11, 12 of the fiber are polished and may be provided with vacuum-evaporation deposited silver coatings (as shown) sothat these end faces constitute internaily reflective termini of aresonant cavity coextensive with the fiber. The coating on the end face12 is made partially lighttransmissive, to permit emission of the laserlight output therethrough. If the fiber is sufficiently long the endcoatings may be dispensed with, since in a long fiber laser the partialinternal reflectivity (typically about 5%) exhibited by uncoated butpolished end faces is adequate to define the resonant cavity. In thelatter case, a laser light output will be emitted through both ends ofthe fiber.

The fiber 10 is surrounded concentrically, along a first portion of itslength, by a. helical gas ous discharge flash tube 15 of a typeconventionally employed to provide pumping light energy for solid-statelasers. The flash tube, energized from a conventional power source 17connected to the end electrodes of the tube through leads 18, 19, isadapted to emit a pulse of light including light in at least oneabsorption band of the laser material. The light-producing discharge inthe tube 16 is initiated, when sufficient charge energy has beendeveloped in the power source, by applying an electrical pulse from acontrol source 20 to a trigger electrode 21 which encircles the turns ofthe fiash tube in proximate relation thereto. A hollow, internallyreflective shield 22 encloses the flash tube and the portion of thefiber 10 surrounded by the tube to contain the pumping light pulseenergy emitted by the flash tube and concentrate this light onto thefiber surface.

It will be understood that the foregoing structures are elements of aconventional unclad fiber laser structure and are adapted to function assuch to produce laser output pulses of intense coherent light. Thus,when a pumping light pulse from the flash tube is initiated as describedabove, this pumping light enters the fiber and excites active atoms ofthe laser material causing them to shift to the upper energy levelrequired for laser action. Assuming for the moment that this provides adegree of inversion in the fiber greater than the threshold value,refiection of spontaneously emitted light through the laser cavity thenstimulates development of a large bidirectionally reflected pulse ofcoherent light (by inducing emissive transitions of upper-level atoms)to initiate a laser output light pulse emerging through the fiber endface 12 and continuing until the depletion of the enlarged upper levelpopulation by such transitions reduces the degree of inversion againbelow the threshold value.

To provide Q control, the device of FIG. 1 further includes a claddinstructure enerall desi nated 23 sura s I: a 7

rounding the fiber 16 along a second portion of the fiber length(external to the shield 22). This cladding structure comprises a body ofliquid Kerr material 24 confined by a capsule 25 and in which the secondfiber portion is in1- mersed. The Kerr material employed is a liquidwhich exhibits 21 Kerrefiect upon application of an electric field, withconcomitant change in refractive index proportional to the square of thestrength of the applied field. Examples of suitable Kerr materials foruse as the cladding 23 are nitrobenzene and carbon disulfidc.

For application of the requisite electric field to the Kerr material 24,a pair of condenser plates 26, 27 are positioned in the body of liquid24, parallel to the long axis of the fiber and in spaced relation to thefiber on opposite sides thereof. These plates are electrically energizedthrough leads 28, 29 extending externally of the capsule 25.

When the plates 26, 27 are not energized, the liquid cladding material24 has a refractive index lower than that of the fiber 10 to providetotal internal reflection at the fiber-liquid interface for lightemitted by the first laser portion. The side walls of the fiber portionsexternal to the cladding 23, being exposed to the air (which also has arefractive index lower than that of the fiber), also have total internalreflection for the laser emitted light. Consequently, throughout thelength of the fiber, energy losses at the side walls are minimized.Light emitted by transitions of upper-level atoms in the laser fiber andimpinging on these side walls at greater than a critical angle ofincidence 6 is concentrated in the fiber rather than being dissipated bytransmission through the walls. Under these low-energy-loss conditionslaser action can readily de velop in the fiber in the manner describedabove.

Energization of the plates 26, 27, however, creates an electric fieldproducing :1 Kerr efiect in the liquid 24 between the plates(immediately adjacent the fiber side walls). With the application of thefield, -the index. of refraction of the Kerr material increases (at asquarelaw rate of change with respect to the voltage of the appliedfield) for the electric light vector parallel to the electric field, anddecreases (again at a square-law rate) for the electric light vectorperpendicular to the electric field. In particular, when a sufiicientlystrong field is applied, the index for parallel-electric-vector lightincreases to a value which is equal to or greater than the refractiveindex of thefiber material. This means that the electric light vector inthe fiber which is parallel to the electric field is no longer totallyreflected at the fibercladding interface, but is partially transmittedtherethrough. The electric light vector perpendicular to the field istotally reficcted', however, as this light undergoes subsequentrefiections, it becomes elliptically polarized, with the result thatduring subsequent reflections, a portion of the light is polarizedparallel to the direction of thefield and hence is not totallyreflected. Therefore, each time the light reflects from the internalsurface of the fiber, its state of polarization is changed'and a portionof the light leaves the fiber, so that after a plurality of reflections6 most of the light will have left the fiber. In other words,application of a sufiiciently strong field lowers the Q condition of thelaser resonant cavity and correspondingly increases the laser thresholdof the structure, since the increased side wall light losses increasethe magnitude of the totality of energy-loss factorsv in the cavity.

In the foregoing system, any application of a sufficiently strongelectric field to the liquid 24 from the start of the pumping periodserves to prevent initiation of laser action. Removal of the field bydeenergization of the lates 26, 27 at a given moment during the pumpingperiod (after establishment of the requisite degree of inversion forlaser action) enables rapid development of a laser light pulse at suchtime. Again, application of a sufficiently strong field to the liquidcladding at a given moment during laser actionv abruptly terminates suchaction, interrupting the laser light output. In this manner, theinitiation and duration of the light output from the fiber can becontrolled by one gization and tie-energizetion of the plates 25, 27.Such control is very rapid, since the response time of the Kerr materialto energization of the plates is of the order of 10- seconds or less, wiereas a single laser output pulse (corresponding to one pumping lightpulse) may ordinarily endure, for example, as long as 10 seconds orlonger.

The magnitude of the output beam spread angle may also be varied, in thestructure of FIG. 1, by controlled energization of the plates 26, 2'7 tochange the refractive index of the Kerr material 24 (for light polarizedparallel to the applied field) within a range of values below therefractive index of the fiber material. Since the fibercladdinginterface is parallel to the axis of the fiber 10, the angle made by anydirection of light propagation with the fiber axis is complementary tothe incident angle of such direction at the interface, enablingmode-selective Q control. Specifically, at any given value of refractiveindex of the Kerr material less than the fiber index, the interface hastotal internal reflection for parallel-electricvector light only inmodes for waves propagating at. an angle to the laser axis which is lessthan the complement p of the critical incident angle 6 As a result thecavity is in high Q condition only for the latter modes; and when theKerr material index for parallel-electric-vector light is raised byincrease in the voltage of the applied field, concomitantly augmentingthe value of 0 the output beam spread angle (which is dependent on 5 iscorrespondingly reduced. In other words the magnitude of the beam spreadangle decreases with increase of the voltage of the applied field.

Various arrangements for controlling energization of the plates 32, 33,to modulate the laser light output with respect to pulse duration, beamspread angle, or

otherwise, by a signal representative of information, will he apparentto those skilled in the art. Thus for example, the plates may beintermittently energ' energized with the aid of appropriate signalcircuitry to provide output pulses of duration controlled to representthe information to be sent. This modulated laser output may be receivedfor demodulation by a conventional detector, as schematicallyrepresented by a photosensitive element 30.

By way of example of a suitable control arrangement for the Kerr cell23, the plates may be energized through a manually operable switch 31from a source of pulse potential 32 which is controlled through acontrol circuit 33 to generate a potential pulse in preselected timerelationship to each trigger pulse generated by the control source 20and used to energize the trigger electrode 21.

The modified structure of FIG. 2 includes a laser cle ment or fiber 36,fabricated as before of a suitable solid laser material, and shown as.having uncoated, polished end faces 37 38 which are sufiicientlyreflective to constitute resonant cavity termini for this elongateelement or fiber. The element or fiber 36 is enclosed for most of itslength in a glass cladding 40, which has a refractive index lower thanthe fiber index so that the walls of the fiber contiguous to thecladding have total internal refiection for the laser emissive light.This cladding 40 is also frequently desirable to provide adequatemechanical strength for the fiber, since the fiber laser core 36 mayoften be of very small cross-sectional diameter and comparatively greatlength. A fiash tube 41 (corresponding to the tube 16 of FIG. 1, butshown for exemplary purposes as a linear rather than a he"cal lightsource) is positioned in proximate parallel re on to a first portion ofthe fiber and the cladding 49 which aids in concentrating pumping lightfrom the flash tube into the fiber core 36), and is arranged to providepumping energy to the fiber, the power source and triggerinstrumentalities being omitted from FIG. 2 for simplicity ofillustration. An internally retlective metallic sleeve 42, correspondingto the shield of FIG. 1, clo e y surrounds the tube 41 and the firstportion of the fiber and cladding.

In the device of FIG. 2, the cladding is polished down or etched away ata second portion of the fiber to expose a surface or side wall 43 of thefiber (as further shown in FIG. 3) for a substantial length. The latterportion of the fiber is immersed in a liquid Kerr material 4" having thesame characteristics and properties as the liquid 24- of FIG. 1. Thisliquid is contained in a capsule #55. A pair of spaced condenser plates47, 4S (analogous to the plates 26, 27 of FIG. 1) are positioned in theliquid 44 on opposite sides of the fiber 36 and parallel to the tlbcraxis, for application of an electric field to the liquid. As shown moreparticularly in FIG. 3, these plates are arranged so that the exposedfiber surface 43 lies in a plane perpendicular to the field createdbetween the plates. The plates are electrically energized throughrespective leads 49, extending externally of the capsule 45, the Kerrcell energizing and control instrumentalities being omitted forsimplicity of illustration.

The arrangement of plates and Kerr liquid in the structure of FIG. 2functions in the same manner as that in the device of FIG. 1 to controllaser action in the fiber 36. Specifically, energization of the plates,by raising the refractive index of the liquid for the laser emissiveencrgy electric vector parallel to the electric field between theplates, renders the fiber surface 42 more light-transmissive, increasingthe energy losses in the laser resonant cavity sufiicicntly to preventor interrupt laser action.

Referring now to FlG. 4, the alternative embodiment there shown-includesan unclad laser element or fiber 51 which has a first portion 53 shapedin the form of a helix to surround a linear flash tube 54. The endsurface 55 of the element or fiber 52 at such first portion isrefiectively coated to provide a first resonant cavity terminus. Aninternally reflective shield 56 surrounds the ficsh tube and the helicalportion of the fiber laser to concentrate light from the flash tube onthe fiber. As -before, the flash tube is associated with appropriatepower source and trigger instrumentalities (not shown) to provide aninput pulse of pumping light for the fiber laser effective to produce alaser light output pulse.

As will be understood by those skilled in the art, due to the so-calledlight-pipe action of an optical fiber body the helical portion 53 of thefiber laser 52 is equivalent to an axially rectilinear fiber in defininga resonant wave-energy propagation path bounded at the unclad side wallof the fiber, which is exposed to the air and thus has total internalreflection. -More generally, it will be understood that the severalflash tube-fiber arrangements of FIGS. 1, 2 and 4 are, for purposes ofthe present invention, equivalent to each other and are thusinterchangeable in the devices of those figures, being shown asexemplary of conventional fiash tube-fiber laser arrangements which maybe employed in the described embodiments of the present invention.

At its second end, the fiber laser 52 of FIG. 4 terminates in an axiallyrectilinear portion 57 having a plane,

unconted end face 58. The structure of FIG. 4 further includes acylindrical hollow cladding 59 formed of a suitable glass or likematerial of given refractive index for laser emissive light energy, andhaving an axially disposed cylindrical passage 60 of diametersubstantially equal to the diameter of the fiber laser 52. This cladding1 is disposed that one end of the pa age 68 is closed by the fiber end53, with the passage pos ion-2d in coaxial relation to the fiber portion57. The other end of the passage is closed by a partially reflectivethin body 62. The passage 60 is filled with a suitable liquid Kerrmaterial 63 (of the type described above in connection with thestructure of FIG. 1), and spaced capacitor plates 64, (respectivelyenergized through lJZtlS 66, G7) are embedded in the member 59 onopposite sides of the passage 6% to provide an electric field effectiveto produce :1 Kerr effect in the material 63; the energizing and controlinstrumcntalities for these plates are again omitted for simplicity ofillustration.

With this arrangement of elements, the resonant propagation path of thecavity structure extends between the fiber end face 55 and the partiallyreflective thin body 62; thus the body of Kerr material 63 constitutes acore portion of the path-defining structure, having the configurationand path-defining characteristics of an optical fiber. Except when underthe influence of an electric field, this core portion of liquid Kerrmaterial has a refractive index higher than that of the surroundingglass cladding 59, providing total internal reflection in the cavity atthe interface between the liquid core and the latter cladding, so thatthe cavity is in high-Q condition. When an electric field is applied tothe Kerr material by energization of the plates 64, 65, however, therefractive index of the Kerr material decreases for the laser lightenergy electric vector perpendicular to the field, until (at asufficiently h gh field voltage) such index becomes equal to or lessthan the refractive index of the surrounding cladding. Total internalreflection then no longer exists at the interface for the perpendicularelectric-vector light component, with the result that energy losses inthe cavity are increased sufiiciently to prevent or terminate laseraction.

Thus the structure of FIG. 4 operates in essentially the same maner asthose of FIGS. 1 and 2 to provide controlled initiation and duration oflaser output pulses or to vary the output beam spread angle, except thatin the FIG. 4 device the core index rather than the cladding index isvaried by application of the electric field. In each of these devicesthe Kerr material exhibits the same index-changing response to thefield, but whereas the devices of FIGS. 1 and 2 are arranged to utilizethe increase in index thus effected for the paralleLelcctric-vectorlight component, the device of FIG. 4 employs the concomitant decreasein index for the perpendicular electric-vector light component, toeffect the desired controlled variation in the Q condition of thecavity.

The arrangement of l lG. 4 is susceptible of various modifications. Forexample, the core portion represented by liquid 63 may be a laserablematerial (and hence an integral portion of the fiber laser 52) whichexhibits an electrically controlled field-responsive variation inrefractive index. Again, a device structurally analogous to that shownin FIG. 4 may be constructed in which both the core portion 63 and thecladding 59 exhibit a fieldresponsive change in refractive index. Insuch case, the core material is selected to have a refractive indexhigher than that of the cladding material when the field is not applied,providing total internal reflection at the corecladding interface. Uponapplication of the field, the refractive index of the core materialdecreases for one elec tric light vector, and the index of the claddingmaterial increases for an electric light vector, until at a sufiicientlyhigh field voltage the latter index becomes equal to or greater than theformer, providing increased light transmission through the core-claddinginterface and thus increasing energy losses in the cavity to prevent orterminate laser action.

The several embodiments of the invention described above and shown inFIGS. 14 rely on fiber optic configurations to provide a surface atwhich energy losses in the resonant cavity can be varied to control thelaser output. That is to say, the changes in core-cladding interfacereflectivity produced in the foregoing structures are effective to alterthe Q condition of the cavity because of the fact that in an opticalfiber the side surface of the fiber provides a path-defining boundary(on which light propagating through the fiber is obliquely incident) andmust therefore have total internal reflection to enable attainment of ahigh-Q condition in the cavity. The same result may be obtained,however, with other optical configurations, as exemplified by thestructures illustrated in FIGS. and 6.

The structure of N6. 5 includes a laser element or fiber 70 (shown asenclosed longitudinally in a glass cladding 71 of refractive index lowerthan the index of the fiber laser material) having a refiectively coatedend face 72 and a surrounding helical fiash tube '73 to provide pumpinglight energy. As before, the supplementary elements shown as associatedwith the flash tube in FIG. 1 are omitted for simplicity ofillustration.

in this embodiment the second end face 74 of the laser body is notrefiectively coated but is instead positioned to abut one face of aprism 76 which is formed of glass having a given refractive index forlaser emissive light energy. A second face of the prism 76 is madeinternally reflective, as with a partially light-transmissive silvercoating 77. Consequently, in this embodiment, the resonant cavitystructure extends between the laser end face 72 and the latter prismcoating. Light emitted in the fiber luster 70 is reflected between theend face 72 and the coating 77 by a diagonal plane surface 78 of theprism, the propagation path of light in the resonant cavity this havinga 90 bend at the diagonal prism surface.

Q control for this structure is provided by 21 Kerr cell generallydesignated '79, including a body of liquid Kerr material 81 intimatelyengaging the diagonal prism surace 78 and contained in a suitablecapsule 82. The Kerr material used is selected to have an index ofrefraction for laser emissive light-energy that is normally lower thanthe refractive index of the prism glass (by an amount providing totalinternal reflection for the laser light incident on the surface '78 at a45 angle), and that can be increased by, application of an electricfield. To create such field, the Kerr cell further includes a pair ofspaced capacitor plates 83, 84 shown diagrammatically as immersed in theliquid Kerr material 81 and electrically energized through leadsrespectively designated 85, 86, by appropriate energizing and controlinstrumentalities (not shown).

When the plates 83, 84 are not energized, the diagonal prism surface 78is essentially totally reflective to light emitted in the fiber laser 79and directed to the surface 78 at an incident angle of approximately 45.Accordingly, the resonant cavity structure is then in high Q condition.Light'can reflect back and forth between the end face 72 and the coating77, and hence laser action can develop (upon pumping of the laster body70) to produce one or a succession of laster output pulses emittedthrough the partially transmissive prism coating '/'7.

Energization of the plates, however, creates an electric field effectiveto raise the refractive index of the Kerr material 81, and thus toincrease the critical angle of incidence for total internal reflectionat the prism surface 78. In particular, when the Kerr material index 21is raised to a value (relative to the index n; of the prism glass) suchthat sin 45 111 the surface 78 no longer has total internal reflectionfor the laser light impinging thereon at an incident angle of 45, butpermits a substantial proportion of such light 13 to pass through thesurface and out of the resonant structure. The energy-loss factors inthe structure are thereby increased sutficiently to create a low Qcondition effective to terminate or prevent laser action.

That is to say, in the structure of FIG. 5 it is not necessary that theindex :1 equal or exceed the index I21 to interrupt or prevent laseraction, but only that the index 11 increase to the aforementioned value.it will be appreciated, moreover, that the 45 incident angle referred toabove is exemplary only, and that (for example) the prism 76 may beshaped so that the laser light from the fiber impinges on the diagonalprisn surface 78 at some other oblique incident angle. In a more generalsense, then, for any given value of the latter angle, laser action canbe switched oli in the FIG. 5 device by raising the Kerr material indexM to a value at which the ratio ri /n exceeds the value of the sine ofsuch angle.

As will now be understood, Q control is effected in the embodiment ofFIG. 5, as in the embodiments of FIGS. 1-4, by means of an electricallycontrolled field effective to vary the refractive index of a claddingmaterial and thereby to vary the internal reflectivity of a clad sidesurface portion (or light propagation path boundary) of the resonantcavity structure intermediate the cavity termini. In function, thedevice of FIG. 5 is essentially similar to the foregoing embodiments. Itwill be appreciated, however, that use of a fiber laser is not essentialto the Q-control function of the FIG. 5 device; that is to say, thelaser included in the structure there shown may be either of the fiberform or of the conventional compare vely large-diameter rcd form.

A modification of the FIG. 5 device, in which the fiber laser 70 isreplaced with a conventional laser rod, is shown in FIG. 6. As thereillustrated, the laser rod (designated 88), formed of asuitable solidlaser material, has a plane, reflectively coated end face 89 and issurrounded by a helical flash tube 90 analogous to the flash tube 73 ofFIG. 5 in arrangement and tuwtion. The second end of the rod 83 isbevelled to pr de a plane uncoated surface 92 at an angle of (forexample) 45 to the axis of the rod. intimately engaging the lattersurface is a body of liquid Kerr material 93, contained in a capsule 94.Capacitor plates 95, 9-5, respectively energized through leads )7, 98,are shown matically as spaced within the capsule to a -y an electricfield to the Kerr material 93. The latter material has a refractiveindex (again designated 12;) lower than the index 12 of the laser rodmaterial when the field is not applied, by an amount providing totalinternal reflection at the surface 2 for light propag ting through therod 88 and impinging on the latter surface at a 45 angle; such light isthen reflected by the suriace )2 between the end face 89 and a plane,partially transmissive reflective coating 9? deposi d on a pl polishedside surface of the rod S3 adjacent the surlxrcc 92, and thus the cavityis in high Q condition, enabling development of a laser output lightpull-ac emitted through the coating 99.

Upon application of an electric field to the Kerr material 93, therefractive index of the latter material is raised (as in the structureof FIG. 5) to a value at which the ratio 11 /11 exceeds the sine of theincidsnt angle of the laser light on the surface 92 (in this case,again, 45). The surface 92 then no longer has total internal reflectionfor such light, with the result that energy losses in the cavity areincreased sufiiciently to prevent or terminate laser action. Thus thedevice of FIG. 6 is essentially similar in structure and operation tothat of FIG. 5, except that the prism 76 of FIG. 5 is omitted and anintegral portion of the rod 83 (that portion including the bevelled endsurface 92 and bearing the reflective coating 99) is arranged to servethe same function as the prism.

The material of variable index included in the structures of FIGS. 1-6as a Kerr material may alternatively be provided by a suitable Faradaymaterial (a material that exhibits a Faraday effect and a change inrefractive index upon application of a magnetic field). By way ofexample, in FlG. 7 there is shown a modified form of the structure ofFIG. 1 incorporating a body of Faraday material in place of the Kerrmaterial 24 as the cladding for the second portion of the laser elementor fiber.

Thus the embodiment of FIG. 7 includes the unclad l' r element or fiber10 of FIG. 1 with internally refiective end faces l1, l2 and fiash tube16 arranged as in FIG. 1 to constitute a conventional fiber laserstructure, certain supplementary elements of FIG. 1 being again omittedfor simpli I of illustration. For the structure of FIG. 7, a portion ofthe element or fiber 10 is clad \viih a body of solid Faraday material154 to pi-ovide Q control operation.

Specifically, the material 154 is selected to exhibit a large Faradayeffect upon application of a magnetic field, and to undergo anaccompanying change in refractive index. A number of types of glasseshave hi h Faraday effects at low temperatures, one suitable materialbeing cerium phosphate glass. To provide the low temperature conditionsrequired for achievement of the desired Faraefiect with such material,the fiber and eiadding 154 may be cooled by appropriate conventionalcooling means (not shown).

As illustrated, the cladding 154 is surrounded by a multi-turn,multi-laycr inductor coil 156, energized through leads 157, 158, byappropriate energizing and control instrunientalities (not shown), tocreate the aforementioned magnetic field. The Faraday cell thusconstituted by this coil and the cladding 154 functions in much the sameway as the Kerr cell of FIG. 1 to effect Q control. In the absence ofthe magnetic field, the refractive index of the cladding is lower thanthat of the fiber laser material and hence the fiber side walls havetotal internal reflection throughout the length of the fiber for laseremitted light. Application of the magnetic field to the cladding 154 byenergization of the coil 156 produces a Faraday effect in the cladding;this Faraday effect results in rotation of the plane of polarization oflight passing through the Faraday material. Concomitantly, therefractive index of the material increases (as a linear function of theapplied field strength) for one electric light vector and decreases(again as a linear function of the applied field strength) for another.If the field strength is sufiiciently high, the increasing index atleast equals the index of the fiber laser material, so that thefiber-cladding interface no longer has total internal reficction for thefirstmentioned laser emissive energy electric vector. This decreases theresonant Q over the clad portion of the fiber walls sufficiently toprevent or terminate laser action. In such manner the initiation andduration of the laser light output are controlled by controllingcnergization of the cl 156.

While the foregoing ember .ents of the invention have been de cribedwith reference to conventional pulsed laser operation, the invention mayalso be employed for laser output control or modulation in so-ealledcontinuous laser operation, wherein the laser body is maintainedcontinuously at a degree of inversion greater than threshold by acontinuous input of intense pumping energy. Thus, for example, byapplication of an electric or magnetic field to a respective Kerr orFaraday cladding of a continuously pumped fiber laser, the otherwisecontinuous creation of laser light output can be initiated andinterrupted to provide light output pulses of controlled duration,controlled time sequence, and controlled amplitude tints to enabletransmission of information.

It is to be understood that the invention is not limited to the specificfeatures and embodiments hcrc nabovc set forth, but may be carried outin other ways without departure from its spirit.

I claim:

1. A laser structure comprising, in combination, means providing aresonant. wave-energy propagation path, said means including an activelaser element having a propagation path portion formed of a firstmaterial transmissive to laser cinissive light energy with a smoothsurface area against which laser emissive light energy propagatingthrough said portion in said path impinges at an oblique angle; a bodyof a second material transmissive to said light energy intimatelyengaging said smooth surface area of said propagation path portion; atleast one of said first and second materials exhibiting for anelectriofield cmnpontat of said light energy an electrically controlledtieldit pvansive variation in refractive index within a range of valuesincluding a value at which the ratio of the refractive index of saidsecond material to the refractive index of said first material for saidlight energy is less than unity; and means electrically energizable toapply thereto a field effective to vary said refractive index of saidone of said materials within said range of values to control the Qcondition of said element.

2. A laser structure comprising, in combination, an elongated andsmooth-sided fiber laser element formed of a material having a givenrefractive index for laser emissive light energy and clad with materialof lower refrac tive index for said light energy except for a length providing an exposed unclad side surface portion, means for energizing saidelement to establish a laser-able inversion of energy states thereof,aconfined body of liquid material intimately ene ing said exposed sideportion of said elcmcr and exhibiting a Kerr effect with correspondingvariation in refractive index for an electricfield component of saidlight energy between lower and upper values respectively below and atleast equal to the refractive index of said element, and a pair ofcondenser plates immersed in said liquid material in spaced parallelrelation to said element on opposite sides thereof and energizable toapply to said liquid material an electrio-field effective to change therefractive index of said liquid material between said lower and saidupper value thereof.

3. The invention according to claim 1 wherein said second materialintimately engages a side surface area of said propagation path portionand exhibits a Kerr effect with corresponding variation in itsrefractive index.

4. The invention according to claim 1 wherein said first materialexhibits an electrically controlled field-responsive variation inrefractive index.

5. The invention according to claim 1 wherein a prism formed of saidfirst material provides a segment of said first path portion, with saidprism comprising said surface, and wherein said second material exhibitssaid electrically controlled field-responsive variation in refractiveindex within said range and specifically between a value below the sineof said oblique angle and a value above the sine of said oblique angle.

6. The invention according to claim 1 wherein said field is a magneticfield.

References Cited by the Examiner UNITED STATES PATENTS 3,208,342 9/1965Nethercot 88--6l FOREIGN PATENTS 674,294 4/ 1939 Germany.

JE\VELL H. PEDERSEN, Primary Examiner.

RONALD L. WIBERT, Examiner.

1. A LASER STRUCTURE COMPRISING, IN COMBINATION, MEANS PROVIDING ARESONANT WAVE-ENERGY PROPAGATION PATH, SAID MEANS INCLUDING AN ACTIVELASER ELEMENT HAVING A PROPAGATION PATH PORTION FORMED OF A FIRSTMATERIAL TRANSMISSIVE TO LASER EMISSIVE LIGHT ENERGY WITH A SMOOTHSURFACE AREA AGAINST WHICH LASER EMISSIVE LIGHT ENERGY PROPAGATINGTHROUGH SAID PORTION IN SAID PATH IMPINGES AT AN OBLIQUE ANGLE; A BODYOF A SECOND MATERIAL TRANSMISSIVE TO SAID LIGHT ENERGY INTIMATELYENGAGING SAID SMOOTH SURFACE AREA OF SAID PROPAGATION PATH PORTION; ATLEAST ONE OF SAID FIRST AND SECOND MATERIALS EXHIBITING FOR AN ELEC-